As cold air is introduced into room air in an air conditioning system it is important to mix them thoroughly to achieve uniform floor to ceiling temperatures. In commercial applications and many residential applications as well introduced air is mixed with the ambient room air by utilizing difusers or air distributors from which the introduced air passes to the room through grilles or slots. Air conditioning systems in which the introduced air volume is made variable by valving means commonly employ a fan-powered box in which air is pulled from the room to maintain needed air velocity and performance in the difuser when only partial cooling is required. Alternatively, induction without a fan in the box has also been used.
Coolness storage by means of ice banks made it possible to lower substantially the temperature of duct air to as much as 43.degree. to 45.degree. F. and this permitted primary fan motors to be substantially reduced in size and energy consumption. It was discovered, however, that fan-powered boxes were necessary to avoid drafts and unacceptable variations in room temperature and after some experience it was concluded that multiple fans in the boxes were more than cancelling the energy savings of the fewer large primary fan motors.
In contrast to the use of multiple difusers and fan-powered boxes in commercial office spaces, it was the practice several decades ago in residential air conditioning systems that cooler introduced air could be mixed with warmer room air by means of high-velocity jets blowing downwardly from jet orifices in the ceiling of a residential room. With these high-velocity air jets the temperature of the emitted air stream would drop close to ambient very quickly and thus eliminate the need for conventional distribution systems. The high-velocity jets were commonly located in the ceiling at the corners of the residential room so that the emitted air stream would be directed into essentially unoccupied spaces.
Variable air volume was achieved in those residential high-velocity jet installations by varying the jet orifice size, and this was done by expansion and contraction of a wax capsule exposed to ambient room air, as taught for example in U.S. Pat. No. 3,235,178. The air streams from the jet orifices had a velocity of approximately 2,000 feet per minute and lost that velocity rapidly as the streams and mixed with ambient air. The temperature and velocity differential was reduced almost by half in the first four orifice diameters away from the orifice and reached zero at twenty orifice diameters away from the orifice. In an eight foot high residential room with two inch diameter orifice the temperature and velocity delta were virtually zero by the time the introduced air flow reached floor level.
Down-flow high-velocity mixing of this type was not acceptable in large commercial rooms because unoccupied corners or other spaces were not normally available and jets blowing air streams downwardly on occupants caused unacceptable drafts. In addition conventional commercial ceilings of removable suspended panels did not lend themselves to ceiling outlets.